When you lift the ring, the marble shoots off in a straight line — not continuing to curve. This single observation reveals one of the most important facts about circular motion: velocity is always along the tangent to the circle. From NCERT Chapter 4 (Exploration edition) Class 9 Science. Aligned with CBSE syllabus 2026-27.
Aim: To investigate the direction of velocity of an object moving in a circular path, and to understand why velocity in uniform circular motion is always directed along the tangent to the circle.
Materials needed:
The ring confines the marble to a circular path. When the ring is present, the marble moves in a circle. When the ring is removed (lifted away), what happens tells us the direction of the marble's velocity at the instant of release.
Before doing the experiment, think it over: When the ring is lifted and the marble is free, which direction do you think it will move?
There are three possible predictions a student might make:
| Prediction | What it would mean |
|---|---|
| (a) The marble continues in a curved path | Its velocity has a curved direction — but velocity is always a straight-line concept |
| (b) The marble moves toward the centre of the circle | Its velocity is directed inward (radially) at the moment of release |
| (c) The marble moves in a straight line along the tangent at its position when released | Its velocity is directed along the tangent to the circle at the release point |
Make your prediction before reading on — this is what the NCERT activity is designed for.
Observation: When the ring is lifted, the marble moves in a straight line — not along the curve of the circle, and not toward the centre. The straight line is tangential to the circle at the point where the marble was when the ring was removed.
When the experiment is repeated with the ring lifted at a different point on the circle, the marble again moves in a straight line — but in a different direction. Each direction is tangential to the circle at the new release point.
Why does this happen?
The marble has velocity at every instant of its circular motion. Velocity is always directed along the direction of motion at that instant — and for circular motion, the instantaneous direction of motion is the tangent to the circle. The ring's wall was providing a continuous inward (centripetal) force that kept bending the marble's path into a circle. The moment the ring is removed, that inward force disappears. With no force to change its direction, the marble continues in a straight line in whichever direction its velocity was pointing at the instant of release — the tangent direction.
This is a direct consequence of Newton's first law: an object continues in a straight line unless acted upon by a net external force. Remove the ring (the source of inward force) and the marble obeys Newton's first law immediately.
NCERT Fig. 4.25 illustrates a circle with several points marked around it. At each point, an arrow is drawn that touches the circle at that point — these arrows are the tangents. The key features of Fig. 4.25 are:
At every position, the tangent — and therefore the velocity — is perpendicular to the radius at that point. This is a fundamental geometric property: a tangent to a circle is always perpendicular to the radius drawn to the point of contact.
Activity 4.5 provides a physical demonstration of what Fig. 4.25 shows diagrammatically. Seeing the marble shoot off tangentially makes the abstract diagram concrete and memorable.
Q. If the marble moves at the same speed throughout, how can it be accelerating?
Acceleration is defined as the rate of change of velocity — not speed. Velocity is a vector: it has both magnitude (speed) and direction. In uniform circular motion, the speed (magnitude of velocity) remains constant, but the direction of velocity changes continuously as the object moves around the circle. A change in direction is a change in velocity. A change in velocity means non-zero acceleration.
This is why uniform circular motion is classified as non-uniform motion (in the sense that velocity is not constant) even though speed is constant. The acceleration in UCM is directed toward the centre of the circle — it is called centripetal acceleration (centre-seeking). This concept is explored further in Uniform Circular Motion (C07).
| Quantity | In Uniform Circular Motion |
|---|---|
| Speed | Constant |
| Direction of velocity | Continuously changing (always tangential) |
| Velocity (vector) | Continuously changing |
| Acceleration | Non-zero, directed toward centre (centripetal) |
| Type of motion | Non-uniform (accelerated) |
Q1. In Activity 4.5, the ring is lifted when the marble is at the topmost point of the circle. In which direction will the marble move? Why?
Q2. A stone tied to a string is whirled in a horizontal circle. The string breaks. In which direction does the stone move?
Q3. An athlete runs around a circular track at constant speed. Is their velocity changing? Is there acceleration? Justify both answers.